1. Technical Field of the Invention
The present invention relates to a fiber-reinforced heat-resistant sound-absorbing material used for the exhaust nozzle and the like of a jet engine and a process for producing the material.
2. Description of the Related Art
Conventional sound-absorbing materials used for the exhaust nozzle and the like of a jet engine have a honeycomb structure. The sound-absorbing material having a honeycomb structure is comprised of a honeycomb made of a heat resistant alloy, a perforated plate, and a rear plate and it has a reactive structure like a Helmholtz resonator, and sound is diminished by the counteractive offset by the reflection, on the rear plate, of sound waves incident from the perforated plate.
In such a sound-absorbing material having a honeycomb structure, the perforated plate, inner honeycomb and rear plate of the material are overheated by a high temperature exhaust gas (for example, from 700 to 800 K or greater) and sometimes change their shape greatly. For example, since the perforated plate is made of a stainless or aluminum panel, the plate may be damaged or deformed by heat, and the brazed portion between the plate and honeycomb may peel.
Furthermore, a sound-absorbing material having the above-described structure can absorb only sound in a narrow frequency band so that noise in a wide frequency band (for example, from 1000 to 4000 Hz) produced by a jet engine cannot be absorbed satisfactorily.
A variety of sound-absorbing materials equipped with both heat resistance and capacity of absorbing sound in a wide frequency band have been proposed (for example, refer to Patent Documents 1 to 3).
According to “a lightweight ceramic sound-absorbing material and a process for producing the same” of Patent Document 1, a sound-absorbing material having both thermal shock resistance and sound-absorbing performance heightened by stacking a dense layer containing ceramic fibers over the surface of a porous foamed ceramic is disclosed.
According to “a lightweight CMC sound-absorbing material and a process for producing the same” of Patent Document 2, the absorbing material is a porous material made of an alumina-based ceramic containing SiC whiskers and having a porosity of from 80 to 92%. It has an improved sound absorption coefficient by controlling its flow resistance by adding silicon carbide fibers without changing the bulk density.
“A porous sound-absorbing material and a process for producing the same” of Patent Document 3 is characterized in that it has a skeleton made of an aggregate having pores and the porous sound-absorbing material has a porosity of from 75% to 85%.    [Patent Document 1] Japanese Patent Laid-Open No. Hei 10-187163    [Patent Document 2] Japanese Patent Laid-Open No. Hei 10-194864    [Patent Document 3] Japanese Patent Laid-Open No. 2002-167290
The lightweight ceramic sound-absorbing material according to Patent Document 1 has many layers of several sound-absorbing materials stacked one after another so that its production cost is high. Moreover, since the sound-absorbing material is manufactured by foaming ceramics and its porosity is therefore not constant, it is difficult to expand the sound absorption effect even to a wide frequency band.
The lightweight CMC sound-absorbing material according to Patent Document 2 is also produced by foaming ceramics and its porosity is therefore not constant so that a satisfactory sound-absorbing effect in a wide frequency band cannot be attained.
The porous sound-absorbing material according to Patent Document 3 is composed of porous ceramics having a porosity of from 75% to 85% so that it is characterized in that it has adequate strength as a sound-absorbing material while maintaining a predetermined sound absorbing effect, and exhibits high sound absorbing effect against noise in a wide frequency band such as noise of a jet engine.
When it is exposed to a high-temperature high-speed exhaust gas, however, the porous ceramic is worn away and at last it loses its function.
As described above, conventional sound-absorbing materials having a honeycomb structure cannot achieve the sound absorbing performance in a wide frequency band which is a property necessary for the reduction of a jet noise and at the same time, when they are exposed to a high-temperature high-speed exhaust gas, they do not fulfill their function because the honeycomb material is damaged by it.
In addition, ordinarily used glass wool has low heat resistance (300° C. or less), while the sound-absorbing materials disclosed in Patent Documents 1 to 3 have problems that in spite of having high heat resistance, they cannot provide stable sound absorbing performance in a wide frequency band which is a property necessary for the reduction of jet noise, are worn away easily by a high-temperature high-speed exhaust gas, and are not light in weight.
The present invention has been made in order to overcome the above-described problems. An object of the present invention is to provide a fiber-reinforced heat-resistant sound-absorbing material which remains undamaged for a long period of time even exposed to a high-temperature high-speed exhaust gas of a jet engine or the like, has a sound absorbing performance in a wide frequency band which is a property necessary for the reduction of jet noise, and is light in weight; and a production process of the material.